Fixation of fractures near an articular surface are often fixed with devices, such as a buttress plate in order to support the articular surface from displacing. These devices share several common features: nearly all are manufactured as a plate which has fixation of the central fragment with one or more bone screws and provide fixation of the peri-articular fragment with one or more supporting tines, posts, or locked screws.
One such device is shown in published US application 20030105461 (Putnam) and comprises a plate with fixed tines that extend at an angle to the plane of the plate. These tines are contiguous with the plate and extend at a fixed angle. A disadvantage of this construction is that the tines are located at fixed positions and the surgeon must implant all of them. Often the geometry of the fracture is such that the location of the tines is not optimally positioned and may enter a fracture site or protrude into the joint.
Another device includes plates that have fixed screws or posts that are screwed into a threaded hole in the plate. Also known, as shown in Orbay U.S. Pat. No. 6,358,250, are plates in which the posts or screws are inserted at varying angles. These constructions require that the head of the post or screw have enough thread purchase on the threads in the hole of the plate; this requires the plate to have sufficient thickness to allow an adequate number of threads for fixation of the post or screw. In addition, these plates require the post or screw to be inserted at a predefined angle.
A further device is shown in Orbay U.S. Pat. No. 6,364,882 and US Publ. Appln. 2002/0143338 wherein the post or screw is provided with a partial spherical head so that it can be placed at a variable angle in the plate, and thereafter secured with a set screw. The spherical head is provided with slots to facilitate its expansion into the hole when it is secured by the set screw. Although this construction allows greater flexibility for inserting the post at a variable angle, it requires even more thickness for the plate in order to compensate for the thickness of the post or screw head in the plate hole as well as providing a sufficient number of threads for purchase by the set screw. In addition, since the set screw only compresses the head of the post or screw over a fairly small surface area, the frictional forces generated are relatively small whereby there is a risk of inadequate fixation of the post or screw and subsequent postoperative angulation and loss of reduction. In addition, this system is cumbersome to use because of the need for the surgeon to manipulate a small set screw behind the post or screw head, resulting in increased difficulty in surgical technique. Finally, because the head of the fixation post is slotted, it creates an area of considerable weakness where the shaft of the post connects to the slotted head. This is subject to considerable torque and is prone to breakage.
Another device is shown in U.S. Pat. No. 5,954,722 (Bono) wherein a split bushing is engaged in a plate and receives a bone screw which expands the bushing and frictionally locks the bushing in the plate. Although this configuration is useful to compress the head of the post to prevent it from backing out, it does not produce a large surface area of contact between the bushing and the surface of the hole. The bushing is narrow and has a spherical radius that is slightly smaller than that of the hole in order to allow initial rotation of the bushing in the hole to orient the screw. As the bone screw head expands the bushing cylindrically, contact of the bushing with the surface of the hole in the plate is primarily restricted to a narrow zone of contact along a single equatorial plane of the bushing since the curvature of the bushing along its principal axis is more pronounced than that of the hole. Although this configuration generates compressive forces on the head of the screw to prevent it from backing out, if forces on the bone fragment generate torque on the post, the resistance to torque is limited to the frictional forces generated from this limited plane of contact of the bushing along the equatorial zone. The device is intended for use in stabilizing multiple vertebrae in the spinal column and the frictional forces are intended to prevent the screws from backing out of the plate but they are not intended to nor are they sufficient to resist torque imposed on the screws by shifting of an unstable bone fragment of a fracture.
Another limitation of the device in U.S. Pat. No. 5,954,722 (Bono) is that the bushing allows variation of the insertion angles within a predefined conical range. However, there are clinical situations in which it is desirable to limited the range of insertion angles primarily along a single plane. For instance, in some situations it may be desirable to allow variation along an axis from proximal to distal, but limit variation from side to side so that the post will not penetrate the joint or other important structures.
Another limitation of the device in U.S. Pat. No. 5,954,722 (Bono) is that the bushing tends to spin as the post head is inserted. As the size of the bearing is reduced, it becomes increasingly difficult to insert the post fully since the bearing spins before sufficient expansion has occurred to generate large frictional forces.